FIG. 7 is a block configuration diagram showing a conventional solar power generation system.
In this figure, reference characters “1a” to “1f” denote respective solar battery panels, reference characters “2a” to “2f” denote respective direct current power lines, and reference character “3” denotes a power conditioner. Each of the solar battery panels 1a to 1f is a panel-shaped product configured to obtain required voltage and current by connecting a plurality of solar batteries in series/parallel, and also referred to as “solar battery module”.
In this system, the solar battery panels 1a to 1f are connected in series or in parallel via the direct current power lines 2a to 2f, and direct-current power generated in the solar battery panels 1a to 1f by solar power generation is supplied to a load via the power conditioner 3. The direct-current power is converted by the power conditioner 3 to alternating-current power.
In introduction of renewable energy for realizing a low carbon society, it is extremely important to diffuse the use of solar power generation. Although it is generally said that the lives of the solar battery panels 1a to 1f are about 20 years, it is difficult to completely eliminate the failure of the solar battery panels since they are industrial products. In addition, it is difficult to find out whether each of the solar battery panels 1a to 1f is in a failure state.
As a result, a monitoring system for the solar battery panels 1a to 1f is required to be introduced without being increased in production and installation cost.
In a solar power generation system of an ordinary house, once the solar battery panels 1a to 1f are installed on a roof, the solar battery panels are rarely subjected to maintenance. Therefore, even when failure of the solar battery panels 1a to 1f is expected on the basis of reduction in output, under current circumstances, it is not possible to specify one or more failed solar battery panels from among the solar battery panels 1a to 1f. As a result of the fact that one or more failed solar battery panels are left untouched in the solar battery panels 1a to 1f, the power to be originally expected to be generated may be lost and the ability of the introduced solar power generation system may not be fully utilized.
If power generation information of the solar battery panels 1a to 1f can be transmitted by utilizing the direct current power lines 2a to 2f for communication with no change, power generation monitoring can be carried out at low cost without providing new communication cables.
However, generally, large noises are generated on the direct current power lines 2a to 2f through which the power generated by the solar battery panels 1a to 1f is being transmitted. As a result, troubles may occur in communication due to the noises, and the communication may not be carried out.
Generally, in a harsh transmission environment, a spectrum spread communication method and error correcting codes increased in redundant bits are employed. A frame-structured transmission data is assembled and transmitted, and re-transmission control (ARQ: Automatic Repeat reQuest) is carried out in a frame unit. Error detection and error correction are carried out in the frame unit in many cases.
As a result, a frame synchronization signal inserted to the top of a frame or a frame synchronization signal decentrally disposed in transmission data in each frame is transmitted for frame synchronization (see Japanese Patent No. 3511520, however, this technique does not use the spectrum spread communication method).
However, if a frame synchronization signal is inserted into transmission data which is short in frame length, transmission efficiency is reduced. For example, if a frame synchronization signal of 16 to 32 bits is inserted into a frame of 64 bits, there is a problem that transmission efficiency is significantly reduced.